Method of heating and electric induction furnace therefor



DIR. E m mm m 2 .N 0 7 T 3 MN M 3 .81 T M E. J. LIMPEL Fi led Dec. 26, 1942 METHOD OF HEATING AND ELECTRIC INDUCTION FURNACE THEREFOR Dec. 21, 1943.

Patented Dec. 21, 1943 UNITED STATES PATENT OFFICE METHOD OF HEATING AND ELECTRIC INDUCTION FURNACE THEREFOR Eugene J. Limpel, Milwaukee, Wis, assignor to A. 0. Smith Corporation, Milwaukee, Wis., a corporation of New York Application December 26, 1942, Serial No. 470,194

' 2 claims. (01. 219-13) This invention relates to method of heating and electric induction furnace therefor and has to do especially with the art of heating tubular quickly the considerablev mass of metal forming the nose of an aerial bomb casing draws a large amount of electric energy, and it has been found that the abrupt changes of load due to switching on and off of such furnaces periodically have caused objectionable fluctuations in the light. circuits connected to the same power distribution lines. And where there are several such furnaces working simultaneously and connected to the same power source the light fluctuations are apt to be quite severe.

The object of the present invention is to prevent abrupt load changes in the operation of induction furnaces,.thus avoiding the objectionable current surges which accompany such abrupt changes.

According to this invention it has been found that by leaving the furnaces continuously on circuit, as opposed to cutting off the current after each heat, and inserting the bomb casing or tube into the furnace gradually and withdrawing it gradually, after heating, instead of inserting and withdrawing it abruptly, the load changes can be effected without distinctly noticeable fluctuations such as would be objectionably reflected in the. lighting circuits. And it has further been found that operation of the furnaces can be so carried out without excessive increase of-power point at which the magnetic core passing axially through the bomb casing must either be interrupted or constricted at or through the aforesaid axial opening.

When this last stage has been reached, it is no longer feasible to utilize a simple stationary magnetic core extending axially of the casing because such a core could not have sumcient cross-sectional area and still be small enough to clear the constricted opening in the bomb cas- It, therefore, is desirable, and probably indispensable, to so design the furnace that the so-called center leg of the core structure, which is situated axially with respect to the bomb casing, be withdrawable from the furnace with the casing. If, however, the center leg is so constituted that its withdrawal is accompanied by an increasing magnetic gap in the magnetic path approximately constant while the bomb consumption and without substantial increase in the operating time per unit.

During the'initial and subsequent early stages of th series of heating operations which are performed in-thetapering of large bomb casings, the axial opening at the apex of the partially tapered casing is-large enough to pass a magneticcore of adequate diameter to conduct all the magnetic flux needed for the entire heating operation; and as long as that condition persists the carrying out of the present invention entails only the problem of devising means well adapted to lift the bomb casing gradually into the furnace heating chamber and thereafter lowering it gradually out of the heating chamber. But as the tapering proceeds, step by step, the

aforementioned axial opening becomes progres-- sively smaller in diameter and finally reaches a circuit of the furnace, it is found that the power input of the furnace drops too abruptly as the bomb casing is lowered, unless the rate of lowering is exceedingly slow.

Accordingly, afurther object of this invention is to prevent inordinately rapid changes in the power input of induction furnaces in those instances where it is necessary to withdraw'the center magnetic leg with the heated article.

The latter object has been achieved by the provision of a novel magnetic core structure wherein the center leg is divided into two or more endto-end lengths and is so supported, co-ordinated and arranged that the component parts thereof move simultaneously and maintain the magnetic casing or other heated article is being withdrawn from the furnace and likewise, preferably, while it is being inserted into the furnace. Although the power input rises and fails, it does so at a sufficiently gradual rate t circumvent noticeable flickering.

Where a group of such furnaces is in use it is preferable to so stagger their operation that when the load is being reduced on one it is being increased on another, By so doing, and keeping the current on continuously, a fairly uniform load on the power supplysystem can be realized and abrupt changes such as would be objectionto be lifted into the heating chamber or, having been heated, is about to be lowered still farther and removed. to a press where a forming operation on the heated nose will be carried out.

The bomb casing, which is identified by reference numeral I 0, is a length of steel tubing of rather large diameter which is tapered at one end to form a nose of approximately paraboloidal configuration. The tapering is accomplished step by step in several consecutive stages, and the nose is heated to a temperature of the order of 2300 F. between successive forming stages.

. Since the casing is made from straight tubing, it follows that the opening at the end to be tapered is of large diameter initially, and it continues through several stages of the tapering process to have a diameter large enough to pass a magnetic core of sufficient cross-sectional area to accommodate all the magnetic flux that is required to effect quickly the intended heating. During those stages the invention is carried out simply by keeping the current on continuously and raising and lowering the casing gradually instead of abruptly relative to the furnace.

Although the current is kept on continuously, the amount of current drawn by the furnace is by no means constant. When the casing is inserted in the heating chamber it functions as a short-circuited secondary coil and, therefore, gives rise to a countermagnetomotive force which neutralizes, in part, the inductive reactance of the furnace and thus causes the input current to increase correspondingly. If the casing were to be lifted rapidly into the heating chamber, the accompanying current increase would be so abrupt as to cause a noticeable flicker in lights connected with the power supply; but if the easing is lifted gradually into the heating chamber, the change of current takes place so gradually that the change in the lighting intensity is not noticeable. The same is true when it comes to withdrawing the casing from the heating chamber. This presupposes that there is enough power available to supply the furnaces without so greatly diminishing the lighting voltage that the reduction of light would be apparent notwithstanding its being accomplished gradually.

As the axial opening in the apex of the taper becomes smaller a point is finally reached where it will no longer pass a magnetic core of suflicient diameter and it then becomes necessary to arrange that part of the core which is situated within the casing so that it can be moved into and out of the heating chamber contemporaneously with the casing. But this gives rise to an additional problem in that even a gradual raising or lowering 'of the movable section would normally cause a too abrupt change of impedance and unless the casing were moved into and out of the heating chamber too slowly for practical purposes would result in noticeable flicker. This problem is dealt with successfully by the present invention as illustrated diagrammatically in the accompanying drawing and explained heremafter.

In each of the figures there is depicted, in addition to the bomb casing l0, an induction furnace II which comprises a magnetic core structure and. coils for energizing the same. The core structure includes an outer leg, portion I2 which is annular and circumscribes the vertical axis of the furnace, a bridging portion I3, 9. lower portion or pole piece I I which circumscribes the vertical axis of the furnace, and a center leg I5 which is composed of an upper portion I6, a

lower portion I1 and an intermediate portion I8 of constricted diameter. Center leg I5 is movable vertically. In Fig.1 it is shown in its elevated position, and in Fig. 2 it is shown in its lowered position.

The axial opening I9 at the apexof the bomb casing is much too small to pass a magnetic core of adequate cross section to accommodate the flux required to perform the heating operation on the whole tapered portion of the casing, and it, therefore, is necessary that the center leg be of a diameter larger than said opening and divided so that the lower portion II can be raised and lowered with the casing and entirely removed from the furnace. In order to provide as much of a magnetic path as possible through opening I9 there is inserted in the upper end of lower portion ll the intermediate portion I8 which is a laminated plug of magnetic material. This may be and preferably is withdrawable from lower portion I1; and it merely makes contact at 20 with the lower end of upper portion I6. The latter is provided at its upper end with a crosshead 2| which is bored at each end to receive guide pins 22 and 23, the lower ends of which are attached to the upper surface of the furnace by means of flanges 24. Pins 22 and 23 serve to center and guide upper portion I6 in its vertical movements. Upper portion I6 and lower portion I7 are of laminated magnetic material; as is the remainder of the core structure.

Casing I 0 is supported at its lower endon a table 25, on which is also mounted a pedestal 25 which supports lower portion ll of the central core. Pedestal 26 need not be laminated'since it does not enter into the magnetic circuit; and

1 it need not be of magnetic material.

Any suitable means, as for example a hydraulic lift comprising a cylinder 2'! and piston rod 28. is provided for raising and lowering table 25. The casing support should preferably be capable of transverse movement to facilitate withdrawing the casing from under the furnace to a. point where it can conveniently be picked up and moved to the press where the hot nose is subjected to a forming operation.

The furnace illustrated is provided with two sets of energizing coils 29 and 30. These encircle the center leg l5 and are designed to produce approximately uniform temperature throughout the tapered nose of thecasing. Low frequency current such as 60 cycle is preferred, and the principal magnetic paths produced thereby are indicated by arrows 3|. o

By reason of alternating magnetic flux passing axially along the center leg I5 within the casing For a more extensive exposition concerning the principles underlying the general design of furnaces like that shown, reference may be had to the copending applications of Edward Bennett,

Serial No. 451,282, filed July 17, 1942, and the present inventor, Serial No. 453,004, filed July 31, 1942.

If the upper portion I6 of the center leg were fixed instead of being movable downwardly, a gap would occur at 20 each time the table 25 started '13 not meant moving downwardly, and the reluctance of that gap would increase so abruptly, as the gap opened, that it would give rise to an abrupt decrease in the current generated in the nose of the casing which, in tum, would cause a correspondingly abrupt decrease in the countermagnetomotive force, and would immediately be reflected in an abrupt diminution of the input current. Conversely, during the upward movement of the bomb casing, a very rapid rise of input current would occur Just before the casing reached the upper limit of its travel, this being due to the rapidly increasing rate of change of reluctance of the gap at 20 and the correspondingly rapid decreae of reluctance at that gap as the gap closes. This could be offset, in part at least, by retarding the rate of up and down movement of the casing at the upper end of its stroke, but that would entail mechanical difliculties and would materially increase the overall time reportion It ha reached a point where the open-' ing of the gap at 20 to diminishi g the circuit.

Assuming that in will have no effect in respect permeance of the magnetic Fig. 2 the casing, together with lower core portion l1 and intermediate core portion I8 is moving upwardly, the intermediate portion is relied upon to lift the upper portion of th center leg; and when the contact is made at 20 there is no abrupt change in the magnetic circuit which would cause a correspondingly abrupt change of input current. Table 25 with the casing thereon can be lifted without hesitation at a speed which may be said to be gradual as distinguished from an abrupt movement; but it to imply that the up and down movement must be exceedingly slow. The faster the movement the more noticeable the change of light intensity will be, and it is best to determine experimentally in each case how rapid the movement may be before the light variation becomes objectionably noticeable. The permissible rate of movement of the casing will vary as between different installations and depends upon the extent of voltage drop across the lighting circuits caused by the furnace or furnaces. The less the voltage drop the greater may be the permissible speed at which the casing may be moved.

While the intermediate core portion I8 i very I undue surges in the current 5 supplying current to the desirable, it is not indispensable to the operativeness of the invention. An air gap could be left in the center leg between the lower end of upper core portion i6 and the upper nd of lower core portion I1, thus eliminating core portion l8, provided suitable means were employed to synchronize the movements of said upper and lower portions in a manner effective to keep the gap of constant width. Such means would possibly entail a direct connection, outside the furnace between table 25 and crosshead 2|, o that portion It would move up and down synchronously with the table.

I claim:

1. The method of operating an electric induction furnace with an outer core member having an opening at one end to receive the end of a tubular article to be heated and a central core member extending axially of the article, comprising keeping the furnace energizing current on continuously throughout a plurality of cycles of operation, moving the article gradually into the furnace at a rate substantially preventing of the power line feeding the furnace while displacing the central core member axially with an auxiliary core mem-. ber inside the article and magnetically in series with said central core member, heating the end of the article in the furnace, gradually withdrawing the article and its inner auxiliary core member from the furnace at a rate substantially preventing undue surges in the current of the power line feeding the furnace while moving the central core leg axially into the furnace to provide a substantially constant reluctance for the core, and successively similarly inserting, heating and withdrawing articles in the course of manufacturing operations.

2. In combination, an electric inductance furnace for heating the ends of tubular articles, comprising an outer core member having an opening at one end to receive the end of the article to be heated and a central core member egtending axially of the article, a power line for gradually inserting and withdrawing the end of the article through said opening at a rate substantially preventing undue surges in the current of the power line feeding the furnace, and

an inner auxiliary core member disposed axially of the article and magnetically in series with the central core member of the furnace, said auxiliary core member and being movable axially in synchronism with the insertion and withdrawal of the article to pro- .vide a core structure of substantially constant reluctance for the furnace.

EUGENE mimmmn furnace, a conveyor for said central core member 

